The basic elements of the separation system are the drive roller and the separation roller (see FIGS. 1 and 20). The drive roller is rotated in a counterclockwise direction (as seen in FIG. 1) by a control motor (not shown) while the separation roller shaft is rotated in the same counterclockwise direction while the separation roller rotation is inhibited by a friction brake mounted on its shaft. When the drive and separation rollers are brought into direct engagement, or when only a single envelope is located in the nip between the rollers, the effect of the friction brake is over-ridden and the separation roller is permitted to reverse its rotation and to move in the same direction as the drive roller at the point of tangency where they contact one another. The separation shaft is spring loaded laterally to create the normal force N between drive and separation rollers. An implementation uses a pair of co-axially disposed rollers (one upper and one lower, on the same shaft) coupled together as a pair of drive rollers and a pair of separation rollers. However, the principal of operation is the same as the single roller illustrated.
In the process of rotating the drive roller with no material in the nip, a drive force F.sub.DR is created as a result of the normal force N and the coefficient of friction between the rollers This drive force F.sub.DR overcomes the resistance force F.sub.RESIST (from the spring brake) on the separation roller allowing the drive and separation rollers to rotate together in the direction of the drive roller at the point of tangency where they meet. This resistance force can also be referred to as the separation force. (See FIGS. 23 and 24)
When a single envelope arrives between the drive and separation rollers the drive force F.sub.DR will be applied from the drive rollers to the envelope and transmitted through the opposite side of the envelope to the separation roller where it overcomes the resistance force F.sub.RESIST of the brake and rotates the drive and separation rollers in the direction of the feeding envelopes. If the resistance force is too high, such that it exceeds the envelope material strength, damage to the mail will result.
If two envelopes arrive together (double feed) between the drive and separation rollers, the drive force F.sub.DR will be applied to the first envelope and transmitted to the opposite side of the envelope. There it overcomes the influence of the friction forces between the first and second envelopes, allowing the first envelope to pass in the direction of feeding. The second envelope is held by the separation force, F.sub.RESIST, where it remains stationary until the first envelope passes through the separation station. This occurs since the friction between envelopes is less than the friction between the rollers and the envelopes. The second envelope is then treated as a single envelope and follows the first Feeders of this type demonstrate excellent results and run significantly better than the rejection requirements established by the U.S. Postal Service.
In order to provide savings in the cost of communications with customers, many organizations are utilizing single or multiple sheets that are merely folded over and fastened at a limited location along the free edges opposite the fold. It has become apparent that a compromise must be made between handling regular material such as envelopes and handling foldovers in the separation system set forth above. While handling regular enveloped material it was found best to increase the separation force as much as the structural integrity of the enveloped material will permit. This provides the fewest doubles delivered since the difference between the separation force and the friction force between envelopes is maximized.